1. Field of the Invention
The present invention relates to a method of inspecting a honeycomb structural body for purifying exhaust gases, particularly a Diesel particulate filter (abbreviated as DPF, hereinafter) for purifying exhaust gases from Diesel engines, and an apparatus used in the method.
2. Related Art Statement
The aforementioned DPF is composed of a porous ceramic honeycomb structural body having partition walls defining throughholes which are alternately sealed at alternate end surfaces of the honeycomb structural body to force an exhaust gas supplied from one end surface to flow out from the other end surface through the partition walls. Thus, DPF can remove fine particulates in exhaust gases by filtration through the porous partition walls. Therefore, in the production process of DPF, the produced DPF has to be inspected to ensure that no defects at internal partition walls, i.e., cracks which are called "internal cracks" and excessively large pores of diameters larger than those required for the filtration are existent, and that no defects at the sealed portions, i.e., non-sealing or insufficient sealing due to gaps formed between the sealing agent and the partition walls and holes formed in the sealing agent, are not existent.
As a method of inspecting the internal defects of such honeycomb structural body, the method of applicant's Japanese Patent Publication No. 62-53,767 has been known wherein the honeycomb structural body is inspected for internal cracks by irradiation thereof from one end surface with a light to project a projection image on a screen arranged at the other end surface, and checking presence or absence of a disturbance of the light permeating through the internal cracks of the partition walls. However, the inspection method was applied to honeycomb structural bodies not having sealing portions at alternate end surfaces, such as catalyst carriers for purifying exhaust gases, so that it is hardly applicable to DPF having throughholes which are alternately sealed at alternate end surfaces of the honeycomb structural bodies, because the inspection light can not permeate through the sealing portions even if the internal cracks are existent on the partition walls. In regard to the defects at the sealing portions, if such defects are small, the amount of the light permeating through the defects is small, so that the inspection of the defects by means of a screen is difficult. If the defects are not projected on the screen, the throughholes have to be inspected one by one, resulting in problems of inefficient and inaccurate inspection.